Re: [PATCH 13/24] alpha: add ARCH_HAS_PTE_SPECIAL support

[Magnus Lindholm]

Hi Matt,

On Tue, Sep 1, 2026 at 5:51 PM Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> Add _PAGE_SPECIAL using bit 19 (0x80000), which is unused in the Alpha
> PTE layout. This enables pte_special()/pte_mkspecial() for marking PTEs
> that are not backed by struct page (VDSO mappings, zero page,
> io_remap_pfn_range, etc.).
>
> Include _PAGE_SPECIAL in _PAGE_CHG_MASK so it is preserved across
> pte_modify() calls.
>
> Assisted-by: Claude:claude-opus-4-6
> Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>

I tested this on an AlphaServer ES40 with
CONFIG_ARCH_HAS_PTE_SPECIAL=y.

A focused anonymous-memory test faulted in 256 shared zero pages,
changed their protections from read-write to read-only and back, and
then wrote to every page to trigger replacement with private COW pages.

The test completed successfully, all data checks passed, and no
page-table or bad-PTE warnings appeared in dmesg.

Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>